1. Field of the Invention
The present invention relates generally to power control apparatuses. More particularly, it relates to a power control apparatus, in which a load such as a motor is connected to a main, circuit, for monitoring an overload or a short circuit thereby to break the main circuit in accordance with an electric current flowing in the main circuit.
2. Description of the Background Art
FIG. 6 is an electrical circuit diagram of a general motor starter. As shown in this figure, a three-phase main circuit 1 is connected to a circuit breaker 2, such as a molded-case circuit breaker or a ground-fault circuit interrupter (an earth leakage breaker), a magnet contactor 3, a thermal relay 4, and a load 13. An ammeter 12 is provided with a current transformer 11 coupled to one phase of the main circuit 1. The circuit breaker 2 serves to automatically break the main circuit 1 when a large current such as a short-circuit current flows or when a leak occurs. Normally, frequent switchings of the flow of a normal current is carried out by the magnet contactor 3 while the circuit breaker 2 is in the closed state.
The magnet contactor 3 connects the main circuit to the load 13 when a closing button 7 is operated. The thermal relay 4 opens a contact 5 while an overcurrent flows for a certain period or more, and then deenergizes an excitation coil (MG) 6 of the magnet contactor 3 to open the main circuit 1. On the other hand, closing the closing button 7 causes a current to flow from a power source 10 through the contact 5 of the thermal relay, the excitation coil 6, the closing button 7 and the opening button 9, and thus causes the excitation coil 6 to be energized and the magnet contactor 3 to be closed. Accordingly, a self-hold contact 8 of the magnet contactor 3 is closed to supply a current to the load 13.
Operation of the opening button 9 causes the circuit to be opened and the excitation coil 6 to be deenergized, and then the magnet contactor 3 is opened, so that a current flowing to the load 13 such as a motor is interrupted.
The individual components such as the circuit breaker 2, the magnet contactor 3, the thermal relay 4 and the current transformer 11 shown in FIG. 6 are provided on a common base and wired therebetween. However, since those individual components are manufactured by different manufacturers, their dimensions and mounting wirings are not the same. Therefore, professional knowledge is required to assemble those components, and thus there is a limitation in miniaturizing the components as a whole in unitary device. Further, for a centralized control of many starters, a starter panel for mounting each starter thereon is required. Further, there is a disadvantage that the conventional starter is unsuitable for a distributed control installation in which each device is controlled individually.